Extended coupled-states approximation for full-dimensional quantum treatments of rovibrationally inelastic scattering...

While the rigorous time-independent close-coupling approach is ideally suited for cold and ultracold rovibrationally inelastic collision, its application beyond atom-diatom systems in full dimensionality is numerically expensive. Coupled-states (CS) approximation and its extensions are good choices to reduce the computational cost and have been successfully applied to diatom-diatom systems. In this work, we introduce the extended CS (ECS) approximation, in which one or a few nearest Coriolis coupled helicity channels are included. Its usefulness in atom-triatom systems is demonstrated for scattering of H₂O with rare gas atoms. The results show that the ECS approximation, even when only the nearest neighbors are included, is generally much better than the CS approximation in describing scattering. At low collision energies, the ECS gradually converges to the exact results with the increasing number of Coriolis coupled helicity blocks. We further discuss three major factors that may lead to the failure of the CS approximation, namely, the reduced mass, collision energy, and triatomic rotational quantum number. It is illustrated that these factors could impact the relative importance of off-diagonal matrix elements in the Hamiltonian, thus influencing the coupling between different helicity channels.